U.S. patent number 6,972,239 [Application Number 10/923,529] was granted by the patent office on 2005-12-06 for low temperature mocvd processes for fabrication of pr.sub.x ca.sub.1-x mno.sub.3 thin films.
This patent grant is currently assigned to Sharp Laboratories of America, Inc.. Invention is credited to Lawrence J. Charneski, David R. Evans, Sheng Teng Hsu, Tingkai Li, Wei-Wei Zhuang.
United States Patent |
6,972,239 |
Li , et al. |
December 6, 2005 |
Low temperature MOCVD processes for fabrication of Pr.sub.X
Ca.sub.1-x MnO.sub.3 thin films
Abstract
A method of fabricating a PCMO thin film at low temperature for
use in a RRAM device includes preparing a PCMO precursor; preparing
a substrate; placing the substrate into a MOCVD chamber;
introducing the PCMO precursor into the MOCVD chamber to deposit a
PCMO thin film on the substrate; maintaining a MOCVD vaporizer at
between about 240.degree. C. to 280.degree. C. and maintaining the
MOCVD chamber at a temperature of between about 300.degree. C. to
400.degree. C.; removing the PCMO thin-film bearing substrate from
the MOCVD chamber; and completing the RRAM device.
Inventors: |
Li; Tingkai (Vancouver, WA),
Zhuang; Wei-Wei (Vancouver, WA), Charneski; Lawrence J.
(Vancouver, WA), Evans; David R. (Beaverton, OR), Hsu;
Sheng Teng (Camas, WA) |
Assignee: |
Sharp Laboratories of America,
Inc. (Camas, WA)
|
Family
ID: |
35430395 |
Appl.
No.: |
10/923,529 |
Filed: |
August 20, 2004 |
Current U.S.
Class: |
438/384;
257/E45.003; 438/240; 438/681 |
Current CPC
Class: |
C23C
16/40 (20130101); H01L 45/1616 (20130101); H01L
45/04 (20130101); H01L 45/1233 (20130101); H01L
45/147 (20130101) |
Current International
Class: |
H01L 021/20 () |
Field of
Search: |
;438/384,240,680,681,785,911,913 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Liu et al., Electric-pulse-induced reversible resistance change
effect in magnetoresistive films, Applied Physics Letters, vol. 76,
No. 19; May 8, 2000, pp 2749-2751. .
U.S. Appl. No. 10/377,244, filed Feb. 27, 2003, Zhuang et
al..
|
Primary Examiner: Nhu; David
Attorney, Agent or Firm: Ripma; David C. Curtin; Joseph
P.
Claims
We claim:
1. A method of fabricating a PCMO thin film at low temperature for
use in a RRAM device, comprising: preparing a PCMO precursor;
preparing a substrate; placing the substrate into a MOCVD chamber;
introducing the PCMO precursor into the MOCVD chamber to deposit a
PCMO thin film on the substrate; maintaining a MOCVD vaporizer at
between about 240.degree. C. to 280.degree. C. and maintaining the
MOCVD chamber at a temperature of between about 300.degree. C. to
400.degree. C.; removing the PCMO thin-film bearing substrate from
the MOCVD chamber; and completing the RRAM device.
2. The method of claim 1 wherein said preparing a PCMO precursor
includes dissolving solid organometallic compounds including
Pr(thd).sub.3 Ca(thd).sub.2, Mn(thd).sub.3, where thd is C.sub.11
H.sub.19 O.sub.2, in organic solvents butylether and tetraglyme,
wherein a 1N metal of each Pr(thd).sub.3 Ca(thd).sub.2,
Mn(thd).sub.3 has a ratio of between about 0.9:0.6:1, and are
separately dissolved in a mixed solvent of butyl ether and
tetraglyme in the volume ratio of 3:1, resulting in a precursor
solution having a concentration of 0.1 M/L of metals for each of
the Pr, Ca and Mn precursors.
3. The method of claim 1 wherein said preparing a substrate
includes preparing a substrate taken from the group of substrates
consisting of Pt/Ti/SiO.sub.2 /Si and Ir/Ti/SiO.sub.2 /Si, and dip
etching the selected substrate in HF having a 50:1 dilution with
water for about 20 seconds.
4. The method of claim 1 wherein the MOCVD chamber is maintained
with an oxygen partial pressure of between about 20% to 30%, the
temperature of the feed line is set between about 240.degree. C. to
280.degree. C., the pressure in the deposition chamber is
maintained at between about 1 torr. to 5 torr, and wherein the
precursor solution has a delivery rate, into the MOCVD chamber of
between about 0.1 ml/min to 0.5 ml/min, for a deposition process
time of between about 20 minutes to 60 minutes.
5. The method of claim 1 which includes annealing the PCMO thin
film at a temperature of between about 500.degree. C. to
600.degree. C. for about five minutes.
6. A method of fabricating a nucleated PCMO thin film at low
temperature for use in a RRAM device, comprising: preparing a PCMO
precursor, including dissolving solid organometallic compounds
including Pr(thd).sub.3 Ca(thd).sub.2, Mn(thd).sub.3, where thd is
C.sub.11 H.sub.19 O.sub.2, in organic solvents butylether and
tetraglyme, wherein a 1N metal of each Pr(thd).sub.3 Ca(thd).sub.2,
Mn(thd).sub.3 has a ratio of between about 0.9:0.6:1, and are
separately dissolved in a mixed solvent of butyl ether and
tetraglyme in the volume ratio of 3:1, resulting in a precursor
solution having a concentration of 0.1 M/L of metals for each of
the Pr, Ca and Mn precursors; preparing a substrate; placing the
substrate into a MOCVD chamber; introducing the PCMO precursor into
the MOCVD chamber to deposit a PCMO thin film on the substrate;
maintaining a MOCVD vaporizer at between about 240.degree. C. to
280.degree. C. and maintaining the MOCVD chamber at a temperature
of between about 300.degree. C. to 400.degree. C.; annealing the
PCMO thin film at a temperature of between about 500.degree. C. to
600.degree. C. for about five minutes; removing the nucleated PCMO
thin-film bearing substrate from the MOCVD chamber; and completing
the RRAM device.
7. The method of claim 6 wherein said preparing a substrate
includes preparing a substrate taken from the group of substrates
consisting of Pt/Ti/SiO.sub.2 /Si and Ir/Ti/SiO.sub.2 /Si, and dip
etching the selected substrate in HF having a 50:1 dilution with
water for about 20 seconds.
8. The method of claim 6 wherein the MOCVD chamber is maintained
with an oxygen partial pressure of between about 20% to 30%, the
temperature of the feed line is set between about 240.degree. C. to
280.degree. C., the pressure in the deposition chamber is
maintained at between about 1 torr. to 5 torr, and wherein the
precursor solution has a delivery rate, into the MOCVD chamber of
between about 0.1 ml/min to 0.5 ml/min, for a deposition process
time of between about 20 minutes to 60 minutes.
9. A method of fabricating a PCMO thin film at low temperature for
use in a RRAM device, comprising: preparing a PCMO precursor;
preparing a substrate, including preparing a substrate taken from
the group of substrates consisting of Pt/Ti/SiO.sub.2 /Si and
Ir/Ti/SiO.sub.2 /Si, and dip etching the selected substrate in HF
having a 50:1 dilution with water for about 20 seconds; placing the
substrate into a MOCVD chamber; introducing the PCMO precursor into
the MOCVD chamber to deposit a PCMO thin film on the substrate;
maintaining a MOCVD vaporizer at between about 240.degree. C. to
280.degree. C. and maintaining the MOCVD chamber at a temperature
of between about 300.degree. C. to 400.degree. C., wherein the
MOCVD chamber is maintained with an oxygen partial pressure of
between about 20% to 30%, the temperature of the feed line is set
between about 240.degree. C. to 280.degree. C., the pressure in the
deposition chamber is maintained at between about 1 torr. to 5
torr, and wherein the precursor solution has a delivery rate, into
the MOCVD chamber of between about 0.1 ml/min to 0.5 ml/min, for a
deposition process time of between about 20 minutes to 60 minutes;
annealing the PCMO thin film at a temperature of between about
500.degree. C. to 600.degree. C. for about five minutes; removing
the PCMO thin-film bearing substrate from the MOCVD chamber; and
completing the RRAM device.
10. The method of claim 9 wherein said preparing a PCMO precursor
includes dissolving solid organometallic compounds including
Pr(thd).sub.3 Ca(thd).sub.2, Mn(thd).sub.3, where thd is C.sub.11
H.sub.19 O.sub.2, in organic solvents butylether and tetraglyme,
wherein a 1N metal of each Pr(thd).sub.3 Ca(thd).sub.2,
Mn(thd).sub.3 has a ratio of between about 0.9:0.6:1, and are
separately dissolved in a mixed solvent of butyl ether and
tetraglyme in the volume ratio of 3:1, resulting in a precursor
solution having a concentration of 0.1 M/L of metals for each of
the Pr, Ca and Mn precursors.
Description
FIELD OF THE INVENTION
This invention relates to processes and fabrication of Resistive
Random Access Memory devices (RRAM), and specifically to a method
to deposit Pr.sub.x Ca.sub.1-x MnO.sub.3 (PCMO) thin film materials
for RRAM applications using liquid delivery MOCVD techniques.
BACKGROUND OF THE INVENTION
Pr.sub.0.3 Ca.sub.0.7 MnO.sub.3 (PCMO) metal oxide thin films,
which show reversible resistance change via applying electric
pulse, have been grown on both epitaxial YBa.sub.2 Cu.sub.3 O.sub.7
(YBCO) and partial epitaxial platinum substrates via pulsed laser
ablation (PLA) technique as described by Shangqing Liu et al., in
Electric-pulse-induced reversible resistance change effect in
magnetoresistive films, Applied Physics Letters, Vol. 76, number
19, pp. 2749, May 2000; and in U.S. Pat. No. 6,204,139 B 1, granted
Mar. 20, 2001, for Method for switching the properties of
perovskite materials used in thin film resistors. In U.S. Pat. No.
6,673,691, granted Jan. 6, 2004, for Method for Reversible
Resistance Change Induced by Short Electric Pulses, a method of
spin-coating PCMO thin film deposition technique is described to
fabricate a reversible resistance switch using a unipolar electric
pulse. In co-pending U.S. patent application Ser. No. 10/377,244,
filed Feb. 27, 2003, for Precursor Solution and Method for
Controlling the Composition of MOCVD Deposited PCMO, a PCMO thin
film, having reversible resistance properties, is formed by
MOCVD.
Liu et al., supra, discovered a resistance change in a PCMO film by
applying bipolar electric pulses at room temperature, specifically
for a Pr.sub.0.3 Ca.sub.0.7 MnO.sub.3 (PCMO) thin film. Liu et al.
deposited PCMO thin films on epitaxial YBa.sub.2 Cu.sub.3 O.sub.7
(YBCO) and on partial epitaxial platinum substrates by pulsed laser
deposition (PLD).
Higher temperature processes result in many problems during process
integration of RRAM devices, such as diffusion, poor thermal
stability of electrodes, etc. Thus, a method of fabricating PCMO
RRAM devices at lower temperature is desirable, in order to
fabricate RRAM incorporating bipolar switching PCMO thin films.
SUMMARY OF THE INVENTION
A method of fabricating a PCMO thin film at low temperature for use
in a RRAM device includes preparing a PCMO precursor; preparing a
substrate; placing the substrate into a MOCVD chamber; introducing
the PCMO precursor into the MOCVD chamber to deposit a PCMO thin
film on the substrate; maintaining a MOCVD vaporizer at between
about 240.degree. C. to 280.degree. C. and maintaining the MOCVD
chamber at a temperature of between about 300.degree. C. to
400.degree. C.; removing the PCMO thin-film bearing substrate from
the MOCVD chamber; and completing the RRAM device.
It is an object of the invention to fabricate a RRAM device having
a PCMO switching thin film.
It is another object of the invention to fabricate a RRAM device
having a PCMO switching thin film at a lower than conventional
temperature.
A further object of the invention is to deposit crystallized PCMO
thin films using enhanced surface nucleation.
This summary and objectives of the invention are provided to enable
quick comprehension of the nature of the invention. A more thorough
understanding of the invention may be obtained by reference to the
following detailed description of the preferred embodiment of the
invention in connection with the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of the method of the invention.
FIG. 2 is an EDX pattern of an as-deposited PCMO thin film
fabricated according to the method of the invention.
FIG. 3 is an EDX pattern of a PCMO thin film fabricated according
to the method of the invention, after a post annealing step.
FIG. 4 is an X-ray pattern of a PCMO thin film fabricated according
to the method of the invention.
FIGS. 5 and 6 depicts bipolar switching properties of a PCMO thin
film fabricated according to the method of the invention.
FIG. 7 depicts bipolar switching properties of a PCMO thin film
fabricated according to the method of the invention after a post
deposition annealing step.
FIGS. 8 to 11 are microphotographs of a PCMO thin film fabricated
according to the method of the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Lower temperature deposition of Pr.sub.x Ca.sub.1-x MnO.sub.3
(PCMO) thin film materials has many advantages for integration of
Resistive Random Access Memory (RRAM) devices. However, only
crystallized PCMO thin films demonstrate RRAM functional properties
with bipolar switching. Therefore, optimized low temperature MOCVD
processes to deposit crystallized PCMO thin films by using enhanced
surface nucleation are advantageous.
The following describes low temperature process 10 for optimization
of MOCVD to deposit crystallized PCMO thin films by using enhanced
surface nucleation, with reference to FIG. 1. The PCMO precursors
are prepared 12 using solid organometallic compounds: Pr(thd).sub.3
Ca(thd).sub.2, Mn(thd).sub.3, where thd is C.sub.11 H.sub.19
O.sub.2. The organic solvents are butylether and tetraglyme. The 1N
metal of each Pr(thd).sub.3 Ca(thd).sub.2, Mn(thd).sub.3 with ratio
of between about 0.9:0.6:1, are separately dissolved in a mixed
solvent of butyl ether and tetraglyme in the volume ratio of 3:1.
The precursor solutions have a concentration of 0.1 M/L of metals
for each of the Pr, Ca and Mn precursors.
Substrate preparation 14 includes forming a substrate of either
Pt/Ti/SiO.sub.2 /Si or Ir/Ti/SiO.sub.2 /Si, and dip etching the
selected substrate in HF having a 50:1 dilution with water for
about 20 seconds.
The substrate is placed in a MOCVD chamber 16, and the precursor
solution introduced into the MOCVD chamber 18 to deposit a
nucleated thin film of PCMO on the substrate. The PCMO precursor
solution is injected into the vaporizer at temperature in a range
of between about 240.degree. C. to 280.degree. C. A chamber oxygen
partial pressure is maintained at between about 20% to 30%, the
temperature of the feed line and vaporizer is set between about
240.degree. C. to 280.degree. C., and the temperature in the
deposition chamber is maintained at between about 300.degree. C. to
400.degree. C., 20. The pressure in the deposition chamber is
maintained at between about 1 torr. to 5 torr. The precursor
solution has a delivery rate, into the MOCVD chamber, set by a
liquid flow meter, of between about 0.1 ml/min to 0.5 ml/min,
preferably at 0.4 ml/min, to form precursor vapors. The deposition
process requires a time of between about 20 minutes to 60 minutes,
depending on film thickness.
A post deposition annealing step includes annealing the PCMO thin
film at a temperature of between about 500.degree. C. to
600.degree. C. for about five minutes in an oxygen atmosphere,
22.
Upon completion of the PCMO thin film deposition, the substrate is
removed from the MOCVD chamber 24, and the RRAM device is completed
according to state-of-the-art processes, 26.
The compositions of PCMO thin films are measured by EDX and phases
of the PCMO thin films are identified using x-ray diffraction.
FIG. 2 depicts the EDX patterns of an as-deposited PCMO thin film,
on a Pt/Ti/SiO.sub.2 /Si wafer, using the lower temperature
deposition processes of the method of the invention. According to
the Pr, Ca and Mn peaks, the PCMO composition is close to
Pr.sub.0.7 Ca.sub.0.3 MnO.sub.3. FIG. 3 depicts the EDX patterns of
a PCMO thin film following a 700.degree. C. anneal for about five
minutes, again, on a Pt/Ti/SiO.sub.2 /Si wafer.
FIG. 4 shows the x-ray patterns of PCMO thin films on
Pt/Ti/SiO.sub.2 /Si wafers using lower temperature deposition and
post deposition annealing processes of the method of the invention.
As is shown in FIGS. 3 and 4, single-phase PCMO thin films having a
weak c-axis orientation are formed.
FIG. 5 shows the bipolar switching properties of PCMO thin films
deposited on Pt/Ti/SiO.sub.2 /Si wafers using lower temperature
deposition processes and programmed with relatively long pulses. As
is shown in FIG. 4, the ratio of higher resistance to lower
resistance is about two orders of magnitude. FIG. 6 shows the
bipolar switching properties of PCMO thin films deposited on
Pt/Ti/SiO.sub.2 /Si wafers using lower temperature deposition
processes and programmed with relatively short pulses.
FIG. 7 depicts bipolar switching properties of PCMO thin films
deposited on Pt/Ti/SiO.sub.2 /Si wafers, after annealing at about
500.degree. C. for about five minutes. In the case of PCMO thin
films annealed at a temperature exceeding 600.degree. C., nearly
all samples evidenced short circuits, while the PCMO thin films
annealed at temperature below 600.degree. C. did not exhibit this
defect. After annealing at temperatures exceeding 500.degree. C.
for 5 minutes, the PCMO thin films demonstrated the improved
switching characteristics, shown in FIG. 7.
FIGS. 8 to 11 depict microstructures of PCMO thin films fabricated
using the low-temperature deposition method of the invention. FIG.
8 depicts an as-deposited PCMO thin film. FIG. 9 depicts a PCMO
thin film after annealing at about 500.degree. C. for about five
minutes. FIG. 10 depicts a PCMO thin film after annealing at about
600.degree. C. for about five minutes, and FIG. 11 depicts a PCMO
thin film after annealing at about 700.degree. C. for about five
minutes.
Thus, low temperature MOCVD processes for fabrication of Pr.sub.x
Ca.sub.1-x MnO.sub.3 thin films have been disclosed. It will be
appreciated that further variations and modifications thereof may
be made within the scope of the invention as defined in the
appended claims.
* * * * *